EFFECT OF VARIOUS PHOSPHORUS AVAILABILITIES ON RADIATION-USE EFFICIENCY IN SUNFLOWER BIOMASS UNTIL ANTHESIS

Sunflower (Helianthus annuus L.) cv. SELECT was grown in 1994 on a 25 year long-term phosphorus (P) field experiment. Phosphorus rates applied annually are: 0 (P0), 17.5 (P1), 35 (P2), and 70 (P4) kg P/ha. The experiment confirms the seasonal trend of Radiation-Use Efficiency (RUE) over the growing period from B4/B5 to F1 stage. RUE values ranges were: P0 (0.8-2.5), P1 (1.0-2.6), P2 (1.2-3.0), and P4 (1.2-3.7) with accumulated IPAR ranging from 25 to roughly 250 MJ/m(2). Sunflower RUE was insensitive to or only weakly sensitive to P nutritional conditions ranging from poor (P0) to moderate (P1). By contrast, RUE of sunflower grown in high (P2) to very high (P4) available P conditions were much higher at any time of the growing period. In soils well endowed in P, it may be worth taking into account this RUE sensitivity for modelling dry matter accumulation in sunflower.